Exhaust-gas temperature raising system for an in-cylinder injection type internal combustion engine

Abstract

An exhaust-gas temperature raising system for a spark-ignition, in-cylinder injection type internal combustion engine includes an electronic control unit. In case that the engine is in an operating condition where the exhaust-gas temperature is required to rise, on an occasion of a main-fuel injection in a compression stroke, the electronic control unit controls engine control parameters such as ignition timing and air-fuel ratio to cause a cool-flame-reaction product, which remains in a combustion chamber in a middle stage or a subsequent stage of an expansion stroke, to have a concentration close to an inflammable concentration limit, and then causes an additional fuel to be injected into the combustion chamber from the fuel injection valve in the middle stage or the subsequent stage of the expansion stroke. The cool-flame reaction of the additional fuel quickly occurs, and the concentration of the cool-flame-reaction product exceeds the inflammable concentration limit, so that the additional fuel is enflamed by self-inflammation. The energy produced by the combustion of the additional fuel is effectively utilized for raising in the exhaust-gas temperature.

What is claimed is: 
     
       1. An exhaust-gas temperature raising system for an in-cylinder injection type internal combustion engine adapted to inject a main fuel directly into a combustion chamber from a fuel injection valve and to spark-ignite the injected main fuel for lean combustion, comprising: engine control means for controlling an engine control parameter such that concentration of a cool-flame-reaction product, produced during a process of combustion of the main fuel and remaining in the combustion chamber at a middle stage or a subsequent stage of expansion stroke, approaches an inflammable concentration limit, when the engine is in an operating condition in which an exhaust-gas temperature is required to rise; and   additional fuel control means for injecting an additional fuel from the fuel injection valve during the middle stage or the subsequent stage of the expansion stroke such that concentration of an entire cool-flame-reaction product, including the cool-flame-reaction product remaining in the combustion chamber and a cool-flame-reaction product newly produced by a cool-flame reaction of the additional fuel, exceeds the inflammable concentration limit, when the the engine control parameter is controlled by the engine control means, whereby a temperature of an exhaust-gas purification device disposed in an exhaust system of the internal combustion engine is raised.   
     
     
       2. The exhaust-gas temperature raising system according to claim 1, wherein said engine control means includes ignition timing setting means for setting an ignition timing at which the main fuel is ignited to a value falling within a range from 10° before compression top dead center to 5° after compression top dead center in terms of crank angle. 
     
     
       3. The exhaust-gas temperature raising system according to claim 2, wherein said engine control means includes air-fuel ratio control means for setting an air-fuel ratio at the time of combustion of the main fuel to a value which is equal to or greater than 25. 
     
     
       4. The exhaust-gas temperature raising system according to claim 1, wherein said engine control means includes air-fuel ratio control means for setting an air-fuel ratio at the time of combustion of the main fuel to a value which is equal to or greater than 35. 
     
     
       5. The exhaust-gas temperature raising system according to claim 1, wherein said engine control means includes at least one of ignition timing setting means and air-fuel ratio control means, and wherein said engine control means controls an ignition timing at which the main fuel is ignited to a value on a delay side relative to an ignition timing at a time when the engine is not in the operating condition where the exhaust-gas temperature is required to rise, or controls an air-fuel ratio associated with the main fuel to a value on a fuel-leaner side relative to an air-fuel ratio at the time when the engine is not in the operating condition where the exhaust-gas temperature is required to rise.   
     
     
       6. The exhaust-gas temperature raising system according to claim 5, wherein when the engine is not in the operating condition where the exhaust-gas temperature is required to rise, the engine is operated in an intake-stroke lean mode in which a target air-fuel ratio is set to a first air-fuel ratio on a fuel-leaner side relative to a stoichiometric air-fuel ratio and in which fuel is injected from the fuel injection valve mainly during intake stroke, or the engine is operated in a compression-stroke lean mode in which the target air-fuel ratio is set to a second air-fuel ratio on the fuel-leaner side relative to said first air-fuel ratio and in which fuel is injected from the fuel injection valve mainly during compression stroke. 
     
     
       7. The exhaust-gas temperature raising system according to claim 1, wherein said additional fuel control means sets a start timing of injection of the additional fuel to a value falling within a range from 70° to 110° after compression top dead center in terms of crank angle. 
     
     
       8. The exhaust-gas temperature raising system according to claim 1, wherein said engine control means sets an air-fuel ratio associated with the main fuel according to engine temperature. 
     
     
       9. The exhaust-gas temperature raising system according to claim 1, wherein said additional fuel control means sets an injection quantity of the additional fuel so that an air-fuel ratio, calculated from a total fuel injection quantity for one cycle, which is equal to a sum of an injection quantity of the main fuel and the injection quantity of the additional fuel, and a quantity of intake air for one cycle, becomes equal to a target air-fuel ratio which varies in dependence on the operating condition of the engine around a specified air-fuel ratio suitable to effectively raise the exhaust-gas temperature. 
     
     
       10. An exhaust-gas temperature raising method for an in-cylinder injection type internal combustion engine adapted to inject a main fuel directly into a combustion chamber from a fuel injection valve and to spark-ignite the injected main fuel for lean combustion, comprising: determining whether the engine is in an operating condition in which an exhaust-gas temperature is required to rise;   controlling an engine control parameter such that concentration of a cool-flame-reaction product, produced during a process of combustion of the main fuel and remaining in the combustion chamber at a middle stage or a subsequent stage of expansion stroke, approaches an inflammable concentration limit, when it is determined that the exhaust-gas temperature is required to rise; and   injecting an additional fuel from the fuel injection valve during the middle stage or the subsequent stage of the expansion stroke such that concentration of an entire cool-flame-reaction product, including the cool-flame-reaction product remaining in the combustion chamber and a cool-flame-reaction product newly produced by a cool-flame reaction of the additional fuel, exceeds the inflammable concentration limit, when said engine parameter is controlled, whereby an exhaust-gas purification device disposed in an exhaust system of the internal combustion engine is raised in temperature.